1. The Field of the Invention
The present invention relates to a method for microstructuring glass substrates, especially flat glass substrates. A substrate surface of the glass substrate is coated with at least one structured mask layer and subsequently exposed to a chemically reactive ion etching process (RIE) with at least one chemical etching gas.
Microstructures provided in glass change the optical properties of the glass. These microstructures are, among other things, required for diffractive optical elements (DOE) and for micro-optical elements, for example in projection and objective systems, in optical filters, in beam formation, as well as beam correction and color correction, but also in fluidics, in micro-reactors and other applications.
2. Related Art
In EP 0 644 462 B1 a method is disclosed, in which the v bn manufacture of microstructures on substrates by reactive ion beam etching (RIBE) takes place. In this method an ion source with an inlet tube for a reactive gas or gas mixture and a cathode is used. Preferably a fluorine-containing gaseous etching agent, for example CF4 or CHF3, is used. Also a mixture of reactive gas an inert gas can be used. However the components and the etching apparatus are exposed to high stresses or loads by reactive ion beam etching. Also the required structures cannot be obtained with justifiable costs by this etching method.
DE 198 44 025 A1 describes different dry etching processes for working optical surfaces and for transferring optical structural elements to optical materials for microstructuring quartz, quartz glass and quartz-containing surfaces. Also a reactive ion etching process (reactive ion etching, RIE) is described, in which CF4, C2F6 or CHF3 are used as principal components of the etching gas mixed with SF6, XeF2, NF3 or CH4, as well as an ion beam etching process for local surface working. The ion energies should be greater than 600 eV, in order to guarantee a sufficiently high sputtering fraction in the etching process. A disadvantageous increase in surface roughness can be avoided by selection of the etching gas. However no etching rates, which are sufficient for current applications, may be obtained by the described methods for some preferred types of glass.
Ezz Eldin Metwalli and Carol G. Pantano, in “Nuclear Instruments and Methods” in Physics Research B 207 (2003), pp. 21-27 have described a method of magnetically enchanced reactive ion etching, MERIE, of silicon-containing and phosphate-containing glass, on the one hand, in a CF4/CHF3-plasma and, on the other hand, for comparison purposes in an argon plasma. Especially it was found that the MERIE method can be superior to conventional RIE regarding etching rate and that the etching rate of glass in fluorocarbon 20 plasmas decreases with increase of chemically removable oxides in the glass. Silicate glass, boron-containing glass and glass with other components have a significantly lower etching rate in comparison to phosphate-containing glass in pure argon plasma. The comparatively higher etching rate with phosphate-containing glass is based on the fact that the etching mechanism is controlled by physical sputtering and thus the comparatively low binding energy on the phosphate glass surface can be overcome. However these results do not apply to the use of CF4 plasma. In that case the etching rate for quartz glass is clearly above the etching rate of other tested glasses. The cause for this is apparently that volatile SiF4 was produced during chemical etching of SiO2. Glasses with ingredients, which produce non-volatile fluorides, have been considered up to now as difficult or impossible to etch, among other things because of poor surface roughness.